INVESTIGATING LUNAR IMPACT BRECCIAS: INSIGHTS FROM THE ALLAN HILLS 81005 METEORITE

Lunar meteorite Allan Hills (ALHA) 81005 holds a unique place in Earth’s meteorite collection being that it was the first meteorite to be identified as lunar in origin. Lunar samples (from Apollo, Luna, and meteorites) not only provide windows into the mineral make-up of our nearest neighbor in space, but yield important insights and constraints on some of the earliest processes in our Solar System, such as the timing of planetary accretion and differentiation, and periods of bombardment. Sample ALHA 81005 has been classified as an Anorthositic regolith (polymict) breccia. To date, low- and high-Ti mare basalt clasts have been identified in addition to granulitic and cumulate breccia clasts, impact melt, clasts of anorthosite (the most common), and clasts of norites and troctolites. Although found and identified in the early 1980s, a comprehensive evaluation of its formation and chronology remains lacking. The composition of ALHA 81005 is consistent with nearside highland rocks of the lunar crust suggesting that its clasts are older than 4.0 Ga.

The primary goal of this study is to examine the clasts and minerals of ALHA 81005 in order to improve our understanding of the timing of the meteorite’s formation, but also the chronology and petrogenesis of its clasts. Back Scatter Electron (BSE) images and preliminary elemental maps were acquired using the Zeiss Supra 35 VP FEGSEM located at the Center for Advanced Microscopy and Imaging (CAMI) at Miami University and a Cameca SX-50 electron microprobe at Notre Dame's Materials Characterization Facility (MCF).

A preliminary study reveals a range of clast types and sizes. The polymict nature of this meteorite is evident and likely records the variety of lithologies that were present at the site of impact. Several clasts appear largely intact, but are highly fractured from (likely) impact and shock. Olivines, exsolved pyroxenes, and maskelynite dominate the mineral assemblage with accessory troilite and taenite identified to date.

Future work will include in-situ electron microprobe analyses of major, minor, and accessory phases. Once appropriate target minerals are located, they will be analyzed (including dating) by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) at the Trace element and Radiogenic Isotope Laboratory (TRAIL), University of Arkansas.